#include<iostream>
#include<cmath>
#include<vector>
using namespace std;
int main()
{
    int count = 0;
    int W,H,n,R;
    scanf("%d%d%d%d",&W,&H,&n,&R);
    vector<vector<int>> sign(W,vector<int>(H)) ;
    vector<vector<int>> arr(n,vector<int>(2));
    for(int i = 0;i<n;i++)
        scanf("%d%d",&arr[i][0],&arr[i][1]);
    for(int x = 0;x<n;x++)
    for(int i = 0;i<=W;i++)
    for(int j = 0;j<=H;j++)
    {
        if(sign[i][j]==0)
        {
            double flag = sqrt(fabs((arr[x][0]-i)*(arr[x][1]-j)));
            if(flag<=R)
                {
                    count++;
                    sign[i][j] = 1;
                }
        }
    }
    printf("%d",count);
    return 0;
}